When the print key of an image forming apparatus that may be a copying machine is depressed, the document set on the document table of the apparatus is irradiated with light and light reflected by the document is projected into the CCD (charge coupled device) of the apparatus. Then, an image signal (analog signal) showing a voltage level corresponding to the density of the projected image formed by the reflected light is output from the CCD.
The image signal output from the CCD is converted into a digital signal by means of an A/D converter to make image data, which is then supplied to an image processing section of the apparatus.
The image processing section appropriately processes the image data input to it and outputs the processed data. At the same time, it also outputs an image clock signal for synchronizing each of the pixels of the output image. The image data and the image clock signal are fed to a pulse width modulating section.
The pulse width modulating section performs a pulse width modulating operation corresponding to the image data. More specifically, a drive signal synchronized for one or more than one pixel of the image data and having a pulse width (high level period) corresponding to the density of the one or more than one pixel, whichever appropriate, is output from the pulse width modulating circuit. When the drive signal is at level High, the laser unit of the apparatus is energized to emit a laser beam.
The laser beam scans linearly the surface of a photosensitive drum along the axis direction of the drum. The linear scanning operation is repeated successively as the photosensitive drum is rotatedly driven. The direction in which the surface of the photosensitive drum is scanned linearly is referred to as a main scanning direction, whereas the direction in which the linear scanning is shifted as the rotation of the photosensitive drum is referred to as the sub scanning direction.
As the linear scanning operation of the laser beam is repeated, an electrostatic latent image corresponding to the document image is formed on the surface of the photosensitive drum. Then, the electrostatic latent image is developed by means of a development unit to produce a visible image. The produced visible image is transferred to a sheet of copying paper.
An IC circuit is typically used for the pulse width modulating section. The IC circuit shows input/output characteristics specific to it, which can adversely affect the quality of the image formed by the apparatus.